This invention relates to electrography and more particularly it relates to magnetic carrier particles and developers for the dry development of electrostatic charge images.
In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is typically achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is generally known to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator which comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternating north and south oriented magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form what is commonly referred to in the art as a "brushed nap". Either or both of the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development, the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060, 4,473,029 and 5,376,492, the teachings of which are incorporated herein by reference in their entirety, teach the use of hard magnetic materials as carrier particles and also apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles. These patents require that the carrier particles comprise a hard magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/gm when in an applied magnetic field of 1000 Oersteds. The terms "hard" and "soft" when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction To Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. These hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased with good image development. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
In the methods taught by the foregoing patents, the developer is moved at essentially the same speed and direction as the electrostatic image to be developed by high speed rotation of the multi-pole magnetic core within the sleeve, with the developer being disposed on the outer surface of the sleeve. Rapid pole transitions on the sleeve are mechanically resisted by the carrier because of its high coercivity. The brushed nap of the carrier (with toner particles disposed on the surface of the carrier particles), rapidly "flip" on the sleeve in order to align themselves with the magnetic field reversals imposed by the rotating magnetic core, and as a result, move with the toner on the sleeve through the development zone in contact with or close relation to the electrostatic image on a photoconductor. See also, U.S. Pat. No. 4,531,832, the teachings of which are also incorporated herein in their entirety, for further discussion concerning such a process.
The rapid pole transitions, for example as many as 600 per second on the sleeve surface when the magnetic core is rotated at a speed of 2000 revolutions per minute (rpm), create a highly energetic and vigorous movement of developer as it moves through the development zone. This vigorous action constantly recirculates the toner to the sleeve surface and then back to the outside of the nap to provide toner for development. This flipping action also results in a continuous feed of fresh toner particles to the image. As described in the above-described patents, this method provides high density, high quality images at relatively high development speeds.
The above-mentioned U.S. patents, while generic to all hard magnetic materials having the properties set forth therein, prefer the hard magnetic ferrites which are compounds of barium and/or strontium, such as, BaFe.sub.12 O.sub.19, SrFe.sub.12 O.sub.19 and the magnetic ferrites having the formula MO.6Fe.sub.2 O.sub.3, where M is barium, strontium or lead as disclosed in U.S. Pat. No. 3,716,630. While these bard ferrite carrier materials represent a substantial increase in the speed with which development can be conducted in an electrostatographic apparatus, many users of such equipment seek even faster development speeds and so further improvements to the carrier and development process are of interest.
U.S. Pat. No. 4,764,445 discloses hard magnetic ferrite carrier particles for electrographic developing applications which contain from about 1 to about 5 percent by weight of lanthanum. As mentioned in this patent, the speed of development in an electrographic process using conventional hard magnetic ferrite materials, while higher than methods using other techniques, such as with soft magnetic carriers, is limited by the resistivity of such ferrite materials. The patent discloses that addition of lanthanum to the hard magnetic ferrite crystal structure in the disclosed amounts results in a more conductive magnetic ferrite particle, yielding greater development efficiency and/or speed of development.
Others have also proposed methods for making conductive carrier particles. For example, U.S. Pat. No. 4,855,206 discloses adding neodymium, praseodymium, samarium, europium, or mixtures thereof, or a mixture of one or more of such elements and lanthanum, to a hard magnetic ferrite material to increase conductivity. U.S. Pat. No. 5,795,692 discloses a conductive carrier composition having a magnetic oxide core which is said to be coated with a layer of zinc metal that is the reaction product of zinc vapor and the magnetic oxide.
Other carriers proposed for use in an electrographic process include multi-phase ferrite composites as taught in U.S. Pat. Nos. 4,855,205; 5,061,586; 5,104,761; 5,106,714; 5,190,841; and 5,190,842.
U.S. Pat. No. 5,268,249 discloses magnetic carrier particles with a single-phase, W-type hexagonal crystal structure of the formula MFe.sub.16 Me.sub.2 O.sub.27 where M is strontium or barium and Me is a divalent transition metal selected from nickel, cobalt, copper, zinc, manganese, magnesium, or iron.
While some of the above-described carriers may have increased conductivity relative to traditional hard magnetic materials previously employed in development of electrostatic images, in many instances the conductivity of the carrier is so great that imaging problems are created due to the carrier being deposited in the image. Although not clear, it is believed that certain levels of conductivity in the carrier can facilitate a flow of charge between the carrier on the nap and the shell, thereby inducing a charge reversal on the carrier and allowing the carrier particles to electrostatically deposit in the image, referred to hereinafter as "image carrier pickup" or "I-CPU". The presence of I-CPU can impact color rendition and image quality.
As can be seen, it would be desirable to develop new carriers that can be used in an electrographic process for the development of latent electrostatic images. It would also be desirable to develop carriers that can exhibit an greater level of conductivity relative to traditional magnetic materials previously employed in such processes, which would provide not only higher development efficiency, but also preferably reduced levels of I-CPU.